Let’s think through the problem. What is a choke?
It’s an inductor. All inductors have reactance that is a function of the frequency in question. The reason that we call a given inductor a “choke” is that it provides a high-impedance relative to the frequency in question. Thus something that might not be a choke at 14MHz is most definitely a choke at 144MHz. At the given frequency, the inductor presents a very, very high impedance to the signal. Placing a shunt between the center conductor and the shield can be characterized as having a low impedance for signals from DC up to some point and then an increasing impedance as a function of frequency. In other words, at DC, it looks like a short. So, when we pass 14MHz, it doesn’t see the choke. It looks like a very big resistor. The amount of static that is dissipated is a function of the ohmic resistance of the choke and the charge on the center relative to the shield. NOW, consider lightning surges of the INDIRECT kind. They are not simple, DC sparks. They have a leading edge and then a 120kHz or so “ring”. It might be a good idea to calculate the inductance needed that will provide a high-impedance to our RF signals, but a low impedance as possible below say, 150-200kHz. In this way, lightning energy flying into our antenna would have a chance to be shunted to the shield/ground as well as DC components. Ultimately, this really becomes a parallel LC network with some of the C being an arc device <grin>. * Xl = 2 pi f L Let’s say we want 1M ohms at 14mHz: L = Xl / 2 pi f = 1,000,000/ 2 pi 14,000,000 = 0.01137 Henry or 113.7 mH Now, what is this value at 120kHz? Xl = 2 pi 120,000 0.01137 = 8572 ohms I’m sure Rick Hiller will check my math. The point is that a given coil has vastly different impedance at decades of frequency difference. B Bill Crowell, N4HPG Pearland, TX [email protected] <mailto:[email protected]> I prefer to live a life of galvanic isolation.
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